1. Field of the Invention
The present invention relates to a control device of an auto-focus mechanism equipped in a camera and an improvement for ensuring rapid and certain focusing.
2. Description of the Background Art
In the case that a zone for measuring a distance between an object and a camera is provided at a picture plane of the camera in an auto-focus mechanism of a contrast method type, a fixed and small zone is conventionally provided at a center portion of the picture plane. Usually, there is photographed a main object positioned at a center portion of the picture plane, so that a lens member is actuated in order to optimize a contrast ratio of a picture at the center portion of the picture plane.
However, when a picture is taken in light, generally a contrast ratio is remarkably reduced at a center portion of the picture. It becomes difficult to find, an optimum contrast in the case of a conventional zone for measuring a distance and therefore its focusing becomes wrong.
In the case of an auto-focus mechanisms in a type of a high frequency detecting method type is one of the contrast methods, an output of a picture image passes through an automatic gain control (AGC) circuit, a high frequency part of a signal of a picture image obtained from the automatic gain control circuit (hereinafter, it is referred as "a high frequency part") is picked up by passing a band pass filter, a picked-up high frequency part is detected in order to detect an evaluative value for focusing (hereinafter, it is referred as "an evaluative value"), and a proper focus position of a lens member where the evaluative value becomes peak is determined. That is, when focusing is proper, a contrast ratio becomes maximum and a high frequency part also becomes maximum. In such cases, one kind of a band pass filer is utilized. Usually, a circuit for amplifying by a fixed gain value is provided at a wave detected circuit in order to increase a sensitivity for picking up the high frequency part. In addition, it is judged whether a peak is over when the evaluative value is going down after the evaluative value is going up. However, a noise is usually included in the evaluative value. Unless the evaluative value changed more than a noise level, it cannot be found that the evaluative value is actually increased or decreased. Therefore, in a conventional method, it is judged whether a displacement of an evaluative value is greater or less than a fixed reference value.
However, in a conventional auto-focus mechanism, there is only one band pass filter. In the case of a narrow band pass filter, if a focus lens is initially positioned far from a proper focus point, a filter output is little changed although the focus lens is moved frontwards or backwards. In this case, a high frequency part is shifted to a relatively low frequency part and an amplitude of the signal is still small. Accordingly, the evaluative value is little changed initially so that it is very difficult to detect whether a frontward direction or a backward direction is a proper direction toward a focus point. Much time is needed to detect a peak of the evaluative value certainly. In the case of a broad band pass filter, a displacement of the evaluative value is changed slightly near the peak, so that it is difficult to detect a real peak certainly.
The above described relation between focusing and an amount of a high frequency part comes into existence in the case that a brightness of an object is constant. If a brightness of an object is changed while a focus lens is moving for detecting a peak of an evaluative value, a wrong evaluative value is recognized as a peak. This occurs since a high frequency part is changed in accordance with a displacement of a brightness.
In the case that an object is dark, an iris is further opened and a gain value of an AGC circuit becomes high. The high gain value of AGC is added to a fixed gain value of an amplifier circuit. Then, a gain value in a whole system becomes very high and a noise component is also increased. If the noise component is large, it is difficult to detect a change of the evaluative value although the evaluative value is increased or decreased a little. A peak of an evaluative value is unclear so that it cannot detect a proper focus position certainly. Further, a noise level is changed in accordance with an opening degree of an iris, a gain value of an AGC circuit and a gain value of an amplifier in a wave detector. It is a problem described as follows, if a reference value for detecting a displacement of an evaluative value is fixed.
(1) In the case of a fixed reference value as a relatively low level, when an iris is relatively closed, a gain of AGC is relatively high or a fixed gain of an amplifier is relatively high, a noise is increased in the evaluative value. A judgement of a peak of the evaluative value becomes unclear due to the noise. As a result, the judgement is sometimes wrong.
(2) On the contrary, in the case of a fixed reference value as a relatively high level, when an iris is relatively opened, a gain value of AGC is relatively low or a fixed gain of an amplifier is relatively low, a peak of the evaluative value cannot be judged until the evaluative value is much decreased, although a noise is relatively less. That is, the peak cannot be detected until a focus lens runs over a proper focus position some distance. A detection of the peak is delayed.
On the other hand, recently, a video camera of a rear focus lens type has been developed, in which a focus lens is provided behind a zooming lens. In the rear focus lens type, a proper focus position of a focus lens is changed depending on a focusing distance of a zooming lens. The focus position is changed as a curved line.
Conventionally, it is difficult to successively change a position of a focus lens in accordance with a focus distance of a zooming lens.